Multifocal spectacle lenses are visual aids used in the management of presbyopia, the condition wherein the accommodative function of the eye is partially or fully lost. The Vision Council of America defines a multifocal as a ‘lens designed to provide correction for two or more discrete distances’. As such Bifocal and trifocal lenses are multifocal lenses and are distinguished from a progressive addition lens, which is ‘designed to provide correction for more than one viewing distance in which the power changes continuously rather than discretely’. The first multifocal lens was the original bifocal lens invented by Benjamin Franklin in 1784. Trifocal lenses were later introduced by John Hawkins in 1826. A bifocal lens typically has one power correction for distance vision and one for near vision, while a trifocal lens typically has one power for distance vision, one for intermediate vision and one for near vision, although there are exceptions to this. Some multifocal lenses are task specific and have a refractive power other than that for distance vision in the upper portion of the lens, or are reversed, with the add power segment in the upper portion and distance vision correction in the lower portion. A quadrafocal lens may take the form of trifocal lens with an added top bifocal segment. Similarly, a ‘double D’ bifocal lens has a 2nd bifocal segment in the upper portion of the lens.
Bifocal segments can have one of several shapes variously termed flat top, curved top, round, rectangular, and ‘Executive’. Trifocals are commonly available in flat top and ‘Executive’ styles. Like bifocals, trifocals segments are available in many sizes and heights. A flat top design is the most common type of bifocal. Segment sizes range from 25 through 40 mm with 28 mm being the most common segment. The curved top bifocal lens is similar to the flat top in general shape, but the top of the segment is curved. The round segment of the ‘Kryptok’ bifocal lens is less noticeable than the curved top bifocal segment. The near vision portion of an ‘Executive’ bifocal lens comprises the entire bottom section of the spectacle lens, providing the largest reading area of all bifocal lenses. In this respect it is most like the original Benjamin Franklin bifocal lens, and is most useful for extended near work requiring a large visual field. Less common today is the Ultex design wherein the segment has a curved upper portion but otherwise comprises the entire bottom section of the spectacle lens like the ‘Executive’ bifocal. Segment tops are generally but not always positioned at or a few millimeters below the lower lid margin, with round segments decentered nasally 2 to 3 mm to allow for convergence.
The majority of bifocal lenses produced today are one piece plastic lenses with the ‘add’ portion integrally formed into the body of the lens. Due to the discontinuous nature of the surface contour where the two sections meet, the segment is visible within the lens and the patient may perceive image jump or image displacement when looking at an object through the different power zones of the lens. Often times there is a noticeable ridge with segments that have a flat top. Fused glass bifocal and trifocal lenses are less common now than years ago. These lenses utilize two compatible glasses with different refractive indices that are fused together at over 600° c. The higher refractive index segment of the fused glass lens as well is visible within the lens and image jump is also present.
No-line or blended bifocals attempt to hide segment visibility by blending the area that joins the different sections of the lens. Blended bifocal lenses have not gained great popularity as the blend area incorporates a curvature inflection or deformation that exhibits blur and distortion.
In addition to bifocal and trifocal lenses, progressive addition lenses are used to provide correction for more than one viewing distance. Unlike bifocal and trifocal lenses, a progressive lens has a gradient of increasing lens power that provides a continuous range of focus between near and far distances. The extent of the power gradient typically ranges from around 12 to 18 mm. Progressive lenses have cosmetic appeal as the segment line and visibility present in bifocal and trifocal lenses is absent, but the useful area of the progressive and near vision zones of the lens is restricted as aberrations resulting from the power change of the lens result in poor vision especially in the lower lateral portions of the lens. As a result, some individuals do not adjust well to progressive lenses and prefer either reading glasses or bifocal or trifocal spectacle lenses.
Prior to the advent of plastic progressive lenses and blended bifocals, designers attempted to develop multifocal lenses with refractive index gradients between viewing portions of the lens to hide the segment line by locally modifying the refractive index at the interface of the segment and carrier lens. Ion exchange methods for segmented glass lenses have been suggested for this purpose, as described in U.S. Pat. No. 3,542,535 to Hensler, for example. The use of inorganic salts to modify the refractive index of a porous glass to create an ophthalmic lens with localized areas of power change has also been suggested. U.S. Pat. No. 4,073,579 to Deeg, et al describes such a lens. None of the lens systems has ever found commercial success, due in part to the difficulty involved in their respective manufacturing processes and the relatively small amount of refractive index change induced. Although some of these methods may be useful for the correction of optical aberrations, a greater amount of refractive index change is required to provide the refractive power needed for the high diopter additions of bifocal and trifocal lenses.
Gradient index plastic lenses produced by diffusion or other methods have been suggested.
U.S. Pat. No. 5,861,934 to Blum et al and U.S. Pat. No. 4,944,584 Maeda et al describe a multifocal lens having a gradient index resin layer between a first resin layer and the second resin layer. The inner portion has a curved segment-like region that provides add power. The refractive index gradient ranges between that of the first layer to that of the second layer in an axial direction and therefore makes the transition between the first and second layers less abrupt and thus makes the finished multifocal zone more invisible. Both of these lenses will exhibit a localized area of plus power and its associated optical properties and aberrations.
U.S. Pat. No. 3,485,556 to Naujokas describes a plastic multifocal lens with a uniform refractive index gradient between upper and lower lens portions of different refractive index. This lens at first appears to be capable of providing the stated distance and near vision properties. A ray tracing of the lens in accordance with the parameters set forth in the patent shows that only when a significantly high plus power configuration is utilized can an add power of even 1 diopter be achieved. Using the refractive indices of 1.5 and 1.6 identified in the patent a calculated plus power of 4.714 diopters is needed in the distance vision portion to achieve the only slightly greater power of 5.714 diopters in the near vision portion, thus the lens is useful only to those needing high plus power correction for distance vision. Furthermore, if a prescription incorporating cylinder power is produced on either the front or back surface, the cylinder power will vary and cause aberrations as a result of the gradually or progressively varying refractive index.
U.S. Pat. No. 6,942,339 to Dreher describes a multifocal or progressive lens constructed with a layer of variable index material, such as epoxy, sandwiched in between two lens blanks. The inner epoxy coating aberrator has vision zones configured to correct aberrations of the patient's eye and higher order aberrations. The variable index coating that comprises the inner layer of this lens does not provide the progressive add power of the lens, rather as stated in the patent it corrects for aberrations of the patient's eye. The lens has many of the limitations typical of aspheric progressive lenses.